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Aquatic humic substances (AHSs) from the eutrophic Lake Kasumigaura, Japan, significantly inhibited the growth of Microcystis aeruginosa at a concentration level similar to that found in the lake. AHSs were isolated and quantified using XAD-8 resin. The effects of these isolated AHSs on the growth of M. aeruginosa cultures were evaluated in a chemically defined medium. The production of siderophores, iron-specific complexing agents, of M. aeruginosa under iron limitation was also examined. Fulvic acid (the lower molecular size fraction of the AHSs) accounted for more than 85% of AHSs in Lake Kasumigaura. Under iron limitation, M. aeruginosa produced hydroxamate-type siderophores; however, they were not effective in enabling substantial recovery of growth. Microcystis aeruginosa growth was significantly inhibited when fulvic acid was 2 mg dry weight·L-1, similar to the concentration in the lake, even when siderophores were produced. Growth inhibition was probably due to a deficiency of iron caused by iron complexation with the fulvic acid. It was suggested that AHSs play an important role in formation and disappearance of M. aeruginosa blooms in eutrophic lakes.

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Abstract:We investigated the characteristics of nutrients (N and P) and dissolved organic carbon (DOC) concentrations during high, intense typhoon storm and base flow periods from three watersheds (W1, W2 and W3) in central Japan. Concentrations in base flow were low and relatively constant. In contrast, concentrations of N, P, and DOC in runoff during typhoon storms became highly dynamic. Particulate nitrogen (PN) and particulate phosphorus (PP) concentrations in runoff were 4-50 times greater than those in base flow for all storm events. Because PN and PP concentrations were significantly correlated with suspended sediment concentrations, particulate nutrient transport may be associated with overland flow in forested hillslopes. Dissolved organic nitrogen (DON), NO 3 -N, DOC and dissolved phosphorus (DP) concentrations in runoff were 1-10 times greater than those in base flow. All nutrients showed a clockwise, discrete hysteresis pattern, which implied that the sources of nutrients were from forested hillslopes. Additionally, the time constant (t 1/e ) calculated from a decreasing function suggested that particulate nutrients were mainly sourced from surface flow, whereas dissolved nutrients were provided from both surface flow and subsurface flow. The ratios of DOC : nitrate and UV : DOC suggested that higher values occurred in the early period of a storm event and was potentially related to the occurrence of overland flow. Our findings suggested that investigation of the parameters (e.g. DOC : nitrate and UV : DOC) were potentially useful for identifying sources of nutrients and potential applications of hydrological separation. Our results also contribute towards understanding the nutrient dynamics during intense storm events when various hydrological processes such as surface and subsurface runoffs govern the nutrient concentrations in headwater streams.

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